1. Field of the Invention
The present invention relates to a battery comprising a cathode, an anode, and an electrolyte.
2. Description of the Related Art
In recent years, many portable electronic devices such as a combination camera (video tape recorder), a mobile phone and a laptop computer have been introduced. Downsizing and weight saving of these devices have been made. Along with these situations, active research and development has been promoted in order to improve an energy density of a battery, particularly a lithium ion secondary battery as a portable power source for these electronic devices.
As a secondary battery coverable of obtaining a high energy density, for example, there is a lithium ion secondary battery, wherein a material such as carbon materials capable of inserting and extracting lithium (Li) is used for an anode. The lithium ion secondary battery is designed so that lithium inserted in the anode material is always in a state of ion. Therefore, its energy density largely depends on the number of lithium ions which can be inserted in the anode material. Consequently, it is thought that the energy density of the lithium ion secondary battery can be further improved by improving an inserting volume of the lithium ions. However, using graphite, which is currently considered as a material capable of inserting and extracting lithium ions most effectively, the theoretical limit of the inserting volume is 372 mAh as a quantity of electricity per 1 g. Lately, the limit is almost attained.
As a secondary battery coverable of obtaining a high energy density, for example, there is also a lithium metal secondary battery, wherein a lithium metal is used for an anode, and precipitation and dissolution reaction of the lithium metal is utilized for anode reaction. The lithium metal has a high theoretical electrochemical equivalent, 2,054 mAh/cm3, and has an energy density equivalent to 2.5 times the graphite used for the lithium ion secondary battery. Therefore, the lithium metal secondary battery has a potential ability to realize a high energy density beyond that of the lithium ion secondary battery.
So far, many researchers have conducted research and development on practical use of the lithium metal secondary battery (for example, refer to Edited by Jean-Paul Gabano, “Lithium Batteries,” Academic Press, London, New York (1983)). However, in the lithium metal secondary battery, its discharge capacity significantly deteriorates after charge and discharge is repeated. Therefore, its practical use is currently very difficult. This capacity deterioration is based on that the lithium metal secondary battery utilizes precipitation and dissolution reaction of lithium metal in the anode. The reason of this capacity deterioration is thought that the deposited lithium metal falls away from the anode, or deactivated by reaction with an electrolyte, in accordance with charge and discharge.
Therefore, applicants of the invention have developed a new secondary battery, wherein an anode capacity includes a capacity component by insertion and extraction of the lithium and a capacity component by precipitation and dissolution of the lithium metal, and the anode capacity is expressed by the sum of the foregoing two capacity components (for example, refer to International Publication No. WO01/22519). In this secondary battery, a carbon material capable of inserting and extracting lithium ions is used for the anode, and the lithium metal is precipitated on the surface of the carbon material during the charge. According to this secondary battery, it is expected that charge and discharge cycle characteristics can be improved while attaining a high energy density.
Conventionally, as an electrolyte for the battery wherein such lithium is used as a battery reacting species, one wherein LiPF6 as an electrolyte salt is dissolved in a carbonic acid ester nonaqueous solvent such as propylene carbonate and diethyl carbonate has been widely used in view of its high conductivity and stable potential.
However, thermal stability of LiPF6 is not satisfactory. Therefore, there has been a problem that when LiPF6 is used for the battery, high temperature storage characteristics or the like are lowered. Such characteristics deterioration arises even when thermal decomposition of LiPF6 occurs slightly in the electrolyte.
As electrolyte salts other than LiPF6, LiBF4, LiCF3SO3, LiClO4, and LiAsF6 are known as well. However, these electrolyte salts have respective problems. That is, LiBF4 has high thermal stability and oxidation stability, but low conductivity, and LiCF3SO3 has a high thermal stability but low conductivity and oxidation stability, leading to a problem that sufficient discharge characteristics cannot be obtained when charged at a high voltage of 4 V or more. Meanwhile, LiClO4 and LiAsF6 have high conductivity, but have a problem that excellent charge and discharge characteristics cannot be obtained. Further, an electrolyte wherein LiClO4 or LiAsF6 is used always has a minor potential. Therefore, there has been a problem that when precipitating a highly reactive lithium metal, these electrolytes react to the lithium metal, leading to deterioration of the capacity. Consequently, in order to resolve the foregoing problems, it has been considered to use a new electrolyte salt.
Meanwhile, an electrolyte salt, which is expressed by a chemical formula of Mn+([BR14]−)n (in this chemical formula, Mn+ represents cation, n represents a valence. R1 represents CxF2x+1 (1≦X≦8)) has been newly synthesized. It has been considered to apply this new electrolyte salt to secondary batteries and capacitors (for example, refer to Japanese Unexamined Patent Application Publication No. 2002-25610, and Japanese Unexamined Patent Application Publication No. 2002-308884).
However, no reports have been received that this new electrolyte salt is used for the battery wherein lithium is used as a battery reaction species as mentioned above. Therefore, practicality of the new electrolyte salt has been unknown.